Spatial distribution of gully head activity and sediment supply along an ephemeral channel in a Mediterranean environment

Catena 39 Ž2000. 147–167 www.elsevier.comrlocatercatena

Spatial distribution of gully head activity and sediment supply along an ephemeral channel in a Mediterranean environment Dirk J. Oostwoud Wijdenes a , Jean Poesen a,b,) , Liesbeth Vandekerckhove a , Maryke Ghesquiere a

Laboratory for Experimental Geomorphology, K.U. LeuÕen, Redingenstraat 16, B-3000 LeuÕen, Belgium b Fund for Scientific Research-Flanders, Belgium Received 4 May 1999; accepted 7 December 1999

Abstract In this study, we examined the factors that control the spatial distribution of bank gully heads along a reach of an ephemeral river ŽRambla Salada. in an area threatened by desertification in Southeast Spain. The activity of 458 gully heads was assessed in the field by pre-defined criteria such as sharp edges, presence of a plunge pool, tension cracks, recent deposited sediments, flow marks, and vegetation re-growth. The results showed that land use has a significant impact on bank gully head activity. Recent land-use changes involving the extension of almond cultivation appears to intensify bank gully head activity. Also, lithology has a clear impact on the bank gully extension. It was further investigated whether the gully heads were important sediment sources that contributed to reservoir sedimentation. The density of Žvery. active bank gully heads along the study reach was one per 17 m of channel length. Average annual retreat volumes were derived from measurements at 46 active gully heads Ž4.0 m3 yy1 .. By selecting all the channel sections in the catchment of the Puentes Reservoir with a similar pattern of bank gullies using aerial photographs, an estimate of basin-wide sediment production of bank gully heads was established. It was estimated that the retreat of active bank gully heads alone in the 12,760 ha study area Žrepresenting 12% of the total catchment area of the Puentes Reservoir. produced 6% of the sediment filling up the reservoir. Considering that the sediment is also derived from other sources such as channel walls, channel beds, and hillslopes, the overall conclusion is that bank gully

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Corresponding author. Tel.: q32-16-326425; fax: q32-16-326400. E-mail address: [email protected] ŽJ. Poesen..

0341-8162r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 4 1 - 8 1 6 2 Ž 9 9 . 0 0 0 9 2 - 2

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expansion in Southeast Spain is a major point source of sediment and therefore, a major process of land degradation. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Gully head erosion; Desertification; Land use; Lithology; Reservoir sedimentation

1. Introduction Many banks of incised ephemeral streams in Southeast Spain show widespread gullying. If the density of gullies is very high, such areas become unsuitable for land use and may, in extreme cases, change into badlands. Moreover, several studies reported that gully erosion produces large amounts of sediments which may significantly contribute to the filling up of local reservoirs ŽPoesen et al., 1996; Wallbrink et al., 1998.. Extension of gullies occurs by headcut erosion, therefore, gully heads can be important sediment sources within a catchment. Thus, in areas threatened by desertification such as Southeast Spain, gully erosion is one of the major processes of land degradation. There are only few studies that specifically quantify the contribution of gully head erosion in sediment budgets of catchments. Oostwoud Wijdenes and Bryan Ž1994. measured the sediment contribution from one gully head in a semi-arid part of Kenya over a nine-storm period and showed that the gully head produced about the same amount of sediment as its contributing catchment area. In order to assess the contribution of gully head erosion to the overall sediment yield in a catchment, the spatial distribution and the rate at which gullies retreat have to be estimated. The objectives of this paper are to determine the factors that control the spatial variation in bank gully erosion and to estimate its contribution to the sediment yield of a reservoir. If such a spatial relationship can be established, these findings can be extrapolated to larger areas with comparable conditions. This will enable the assessment of the contribution of gully head extension on a catchment scale to the sedimentation rate of the reservoirs. Since bank gullies are by definition confined to the fringes of the Žephemeral. channels, the rate of catchment-wide sediment production is determined by the density of bank-gullies along these incised channels. Factors that control the activity Ži.e. retreat by erosion. of gully heads include land use Žrunoff production., lithology Žrunoff production and erodibility. and topography.

2. Study area The study area is located in the upper part of the Guadalentin catchment, Southeast Spain ŽFig. 1.. The Rambla Salada, an ephemeral channel in Southeast Spain, is a typical example of an incised channel whose banks are intensively gullied ŽFig. 2.. Reservoir sedimentation is a serious problem in this region, which is threatened by desertification ŽPoesen and Hooke, 1997.. Land-use changes during the last decades concerned mainly an increase in almond trees’ plantations and a decrease in matorral ŽPoesen and Hooke, 1997.. The bank gullies develop into highly erodible sedimentary deposits including Tertiary marls and conglomerates, and Quaternary fills. The predominantly silt loam texture of the latter closely resembles that of the marls from which they

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Fig. 1. Location of Guadalentin and Rambla Salada.

are derived. However, locally, these deposits are more sandy and gravelly. The silt loams occupy depressions and channels from an earlier phase of incision. Also present are resistant banks of conglomerates and petrocalcic horizons Žcaliches.. The gullies extend only into the lower pediments and not into the steepest parts of the landscape.

Fig. 2. Overview of Rambla Salada. The banks of this ephemeral stream are dissected by gullies.

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This is typical for bank gullies ŽPoesen et al., 1996., which require a base level lowering for their initiation, but once initiated, runoff is the critical factor which determines their development. The overall activity of gullies is thus largely controlled by the runoff generating potential of the contributing catchment area. Therefore, on the long-term, the largest bank gullies develop into the largest catchment areas ŽVandekerckhove et al., 1999.. This can also be observed along the study reach where large gully systems have developed into thalwegs of dry valleys, which may be sites of historical gullying. However, locally, gully head erosion may show considerable variety, related to local conditions such as the erodibility of the lithology and the land-use type. The area around the town of Zarcilla de Ramos ŽFig. 1. consists of inter-mountain sedimentary basins with steep calcareous mountain ridges surrounding broad, incised, valley bottoms ŽFig. 2.. The footslopes form long pediments, which often are protected by petrocalcic horizons. The upper slopes are wooded or covered by a semi-natural Mediterranean shrub Žmatorral. vegetation. The pediments are mostly cultivated with wheat and almonds, while some of the fields are Žtemporarily. abandoned. Large areas along the Rambla are covered by a grass species, Stipa tenacissima (alfa grass), which develops large tussocks. Grazing by sheep and goats occurs mainly in the matorral and in the abandoned fields. The source area of the Rambla Salada is located in the Sierra de Almirez, in the northwest of the Guadalentin basin. A 3.5 km long reach of the Rambla Salada was selected to the west of Zarcilla de Ramos ŽFig. 1., since it shows the typical features of bank gully development along Ramblas in Southeast Spain. The channel width and depth vary but straight sections are about 10 to 15 m wide and 6 to 10 m deep while outer banks can be up to 12 m high. On its southeasterly trajectory, the channel cuts through thick Tertiary and Quaternary deposits which include marls Žmostly Miocene., conglomerates, and gravelly sandy loams ŽQuaternary.. The area has a Mediterranean type of climate with an average annual rainfall of 276 mm and a mean daily temperature of 14.98C ŽCasa Forestal Zarcilla de Ramos, Andrade, 1990.. 3. Methods All bank gully systems that were connected to the selected reach of the Rambla Salada were mapped in the field in September 1997. Prints of digital aerial photographs Table 1 Criteria used for classifying gully head activity Active

Not active

Sharp edges Plunge pool Undercut Tension cracks Recently deposited sediment Flow marks Piping

Rounded edges No plunge pool Inclined gully head wall Vegetation on gully walls and bed Extremely small contributing catchment area

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Fig. 3. Sketch of gully head that shows signs of activity.

at a scale of 1:1500 were used as a base map. The digital photos had a resolution of 60 cm and were flown about 5 months before. The activity of each gully head was assessed by using a set of pre-defined criteria which are listed in Table 1 and Figs. 3 and 4. On the basis of the assessment, a gully head was classified as being very active, active, moderately active or not active. Gully heads that were classified as not active were characterised by rounded walls, partly overgrown with vegetation and with no overland flow marks ŽFig. 5.. Moderately active gully heads had some flow marks or a small channel through which, occasionally, water flowed. In some cases, rather straight walls were well preserved in resistant material. If recently deposited sediment was found at the foot of these walls or if the headwall was strongly undercut, it was also classified as

Fig. 4. Sketch of gully head that shows signs of inactivity.

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Fig. 5. Example of gully head with dense vegetation growth which was classified as not active.

moderately active. Tension cracks indicate activity but when they occur on the sidewalls, they are indications of widening and not so much of retreat. Those gully heads

Fig. 6. Example of active gully head in zone with S. tenacissima Žalfa grass..

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were also assessed as being moderately active. If the tension cracks occur at the head due to undercutting they indicate an active gully. However, tension cracks do not develop quickly when there is no strong undercutting of the wall. Hence, such gully heads are active but they were not classified among the most active since it was assumed that retreat by tension cracks is relatively slow compared to retreat by concentrated

Fig. 7. Example of active gully head in almond plantation.

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overland flow. Active gully heads had sharp edges and little vegetation below the head. Usually, flow marks were visible such as a plunge pool or an active channel or incision above the headcut ŽFigs. 6 and 7.. Very active gully heads had very sharp edges, freshly exposed gully head walls, and clear flow marks. At 55 gully heads, control measures were applied by farmers. These measures usually consisted of small earth dams around the gully head of about 0.30 m high. The land use and vegetation structure Žland cover. was also mapped in the catchment areas of the bank gullies. Both the gullies and the land cover field maps were copied on to transparency paper which overlaid 1:5000 orthophotos of the area. These products were then converted into digital format for analyses using a GIS. A digital lithology map was already present, however, it was not detailed enough for our mapping purposes since it frequently conflicted with our own field interpretation of the lithology. Therefore, the digital lithology map was not used for any GIS analysis. Instead, we used our own field interpretations which were limited to gully sites only. The second stage involved the estimate of catchment-wide sediment production due to gully head erosion. Monitoring of gully head erosion by repeated field measurements in the catchment started in April 1997 ŽVandekerckhove et al., in preparation.. A network of benchmark pins was installed at 46 sites in Southeast Spain. The pins Žat least two near one gully head. allowed set-up of a base line from which the distance to the sharp edge of the gully head was measured at 10 cm intervals on average. The planimetric difference between two sets of measurements provides an aerial extent of gully head erosion. By multiplying this surface with the average height of the gully head, a volume is obtained Žfor details, see Vandekerckhove et al., in preparation.. An average eroded soil volume value per gully head is then calculated from the average of the erosion volumes of the 46 gully heads. For this study, only 2 years of data are available, which is a rather short period. Nevertheless, this figure is used in this study in order to make a first assessment of sediment production by retreat of active bank gully heads. 4. Results 4.1. Density of gully heads along the selected Rambla reach A total of 458 gully heads were mapped, 208 on the left bank and 249 on the right bank. These included some sites where no individual headcuts could be identified but a complex of usually active channel heads that merged into one larger system. Table 2 shows that 205 gully heads, or 44.7%, is active or very active. This corresponds with a density of one Žvery. active gully head per 17 m of Rambla length. 4.2. Lithology As mentioned before, field interpretations of the lithology in which a gully head occurred often disagreed with the map units from the available geological map. This was mostly due to the scale of the geological map Ž1:50,000, IGME, 1976., which was not detailed enough for our 1:1500 field mapping. The silt loams ŽQuaternary fills. often occupied older channels which were limited in width and therefore, not indicated on the

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Table 2 Total number of gully heads and percentages per activity class. The density of gully heads is expressed as gully head per meter of Rambla and as its reciprocal value, the mean length of Rambla with one gully head Gully head type

Number of gully heads

Gully head per meter of Rambla bank Žmy1 . a

Length of Rambla bank with one gully head Žm. a

Total Very active Active Moderately active Not active

458 Ž100%. 39 Ž8.5%. 166 Ž36.2%. 150 Ž32.8%. 103 Ž22.5%.

0.13 0.01 0.05 0.04 0.03

7.6 89.7 21.1 23.3 34.0

a

Total length of surveyed Rambla section equals 3500 m.

map. These filled in channels were also sites where new gullies developed. According to the geological map, the aerial distribution of lithologies in the delineated research area Ž5.22 km2 . included mostly gravelly sandy loams ŽQuaternary fills, 45%. and marls Ž32%.. The rest consisted of conglomerates, Žpetrocalcic horizons, 14%., gypsum-rich clay deposits Ž8%. and others Ž1%.. We considered the name gypsum marls more appropriate than gypsum-rich clays and used it instead. On the basis of our fieldwork, we distinguished the following lithologies: conglomerates, gravels Žusually present as banks in Quaternary deposits., Žsandy. silt loam ŽQuaternary fills. and marls. The marls differed substantially in appearance and could be subdivided into: marls Žcommon, grayish., gypsum marls Žpresence of gypsum crystals., calcareous marls Žvery white. and red marls Žvery red.. The Quaternary fill dominantly consisted of brownish sandy loam and loams, sometimes including gravelly layers. Table 3 shows the distribution of gully heads and their activity over the lithological units in absolute counts ŽTable 3a and c. and the total percentage of gully heads per unit ŽTable 3b and d.. Most gully heads, 61.4%, occur in Quaternary fills but this was expected since these deposits are so widespread. Therefore, Table 3b presents the same data for each lithological unit but this time normalized to 100%. The tables show that relatively, the most very active gully heads occur in the marls. Quaternary fills have the most active gully heads. As expected, the gully heads in the conglomerates are the least active. If the subdivision of the marls is taken into account, large variation can be viewed. The gypsum marls include the least active gully heads while the calcareous and red marls contain together relatively the most very active and active gully heads. A Pearson’s Chi-square test ŽWalford, 1995. was applied to test the independence between the lithology and the activity of gully heads. The test statistics ŽPearson’s x 2 . is the sum of the squared differences between the observed Ž O . and the expected Ž E . frequencies in each class divided by the expected frequency Ž x 2 s SŽŽ O–E . 2rE ... Since the expected Žreal. distribution is not known, it is calculated from the row and column totals of the cross-tabulation ŽTable 3.. Thus, the expected frequencies are the counts that would occur in each cell of the table if the observations were distributed across all cells in proportion to the row and column totals. When the degrees of freedom Ž df . are determined Ž df s Žrows–1.Žcolumns–1.. the calculated x 2 can then be compared with a

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Table 3 Ža. Cross-tabulation of lithological units and gully head activity classes. x 2 s Pearson’s Chi-square statistic Žno’s.; df sdegrees of freedom. Žb. Gully head activity expressed as a percentage of each lithological unit. Žc. As Ža., but Marls are subdivided into four groups, note that the x 2 test indicates a stronger relationship between gully head activity and lithology. Žd. Gully head activity expressed as a percentage of each lithological unit Lithology

Very active

Active

Moderately active

Not active

Total

(a) Absolute counts Conglomerate 0 2 4 4 10 Gravel 1 13 31 20 65 Quaternary fill 24 117 79 61 281 Marls 14 34 36 18 102 Total 39 166 150 103 458 x 2 s SŽŽobserved–expected. 2 rexpected.; example: expected counts of very active gully heads in Marlss Ž102=39.r458s8.7; x 2 s 26.1; df s9; significant at 0.05 level (b) Percentage of total Conglomerate Gravel Quaternary fill Marls

0 2 9 14

20 20 42 33

40 48 28 35

40 31 22 18

(c) Absolute counts Conglomerate 0 2 Gravel 1 13 Quaternary fill 24 117 Marls 7 24 Gypsum marls 0 4 Calcareous marls 4 2 Red marls 3 4 Total 39 166 x 2 s 57.8; df s18; significant at 0.001 level

4 31 79 20 13 2 1 150

4 20 61 11 6 1 0 103

40 40 48 28 32 57 22

40 31 22 18 26 11 0

(d) Percentage of total (no’s) Conglomerate 0 Gravel 2 Quaternary fill 9 Marls 11 Gypsum marls 0 Calcareous marls 44 Red marls 38

20 20 42 39 17 22 50

100% 100% 100% 100%

10 65 281 62 23 9 8 458

100% 100% 100% 100% 100% 100% 100%

critical value for a desired probability level. This critical value is the upper limit below which the difference between the observed and the expected frequency counts is statistically not significant and has arisen merely through chance. In our case x 2 s 26.1 and df s 9, which according to published tables ŽWilliams, 1984. is significant at the 0.005 level. Thus, the results indicate that the activity of the gully heads is strongly dependent on lithology. When the marls are subdivided ŽTable 3d., the Chi-square test suggests an even stronger dependency of gully–head activity on lithology Ž x 2 s 57.8; df s 18; significant at 0.001 level..

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The coincidence of Žhigh. activity of gully heads and marl deposits confirms the often cited erodibility of marls. This has been attributed to the dispersion of the clay minerals, high swelling pressures Žclosure of pores., and slaking ŽBryan and Yair, 1982.. It is also well-recognised that badlands in the Mediterranean often occur in Žsaline and alkalinesilty. marl deposits ŽYassoglou 1995, Poesen and Hooke, 1997.. The low activity of gully heads in the gypsiferous marls might be due to the formation of water-stable aggregates because of the presence of Ca2q. In addition, at some sites, the gypsum crystals formed almost a protective cover Žerosion pavements. on the soil surface. The conglomerates are clearly the most resistant; they only erode by undercutting due to erosion of underlying layers. 4.3. Land coÕer The distribution of land-use types in the research area is shown in Fig. 8. The largest parts of the delineated area are taken in by wheat fields, 34%, and almond groves, 33%. Stipa occupies 24%, of which 9% occurs on slopes adjacent to the Rambla or other large gullies and 15% are higher grounds such as hilltops. Abandoned fields contain 4% of the area and vineyard 1%. The other 4% includes space taken up by the Rambla and roads. Table 4 shows land cover types and gully head activity. Of all gully heads, 70% have their catchment areas in Stipa. This is because most areas adjacent to the Rambla are covered by S. tenacissima ŽFig. 8.. However, if we consider the gully head activity in each land cover type separately ŽTable 4b., it appears that the most active gully heads occur in the cultivated fields, especially in almond groves. This suggests that the cultivated fields produce more runoff andror the soils are more erodible. The Chi-square test Ž x 2 s 18.7; df s 12. confirms only a linkage between gully head activity and land-use type at the 10% confidence level which indicates that the relationship is not very strong ŽTable 4.. The poor relationship could be due to the control dams which were probably built at a particular gully head because it was active. To test this hypothesis, a new cross table was created by adding moderately active and non-active gully heads with a control dam to the group of active gully heads ŽTable 4c.. The Chi-square test was again used and indicates a much stronger relationship between gully head activity and land use after adjusting the data for gully control measures Ž x 2 s 25.6; df s 12; significant at 0.02 level.. The data also indicate that gully heads in almond groves are the most active, followed by those in wheat fields. Abandoned fields and stipa areas include less active gully heads. This pattern could signify a hierarchy in runoff generation, however, no data are available about this pattern. Therefore, the curve number method ŽCN. was used to verify these findings. Curve numbers are indices for runoff production that can be used for runoff estimation from small catchments ŽSoil Conservation Service, 1986.. Although the system was developed for North America, equivalent CN-values for the Mediterranean area are provided by Lopez Cadenas et al. Ž1994.. The following curve ´ numbers were deduced from this source: almond groves: 88, Žstraight rows and assuming poor hydrological conditions and slow infiltration rates Žhydrological soil group C.., wheat fields: 84, Žstraight rows and assuming poor hydrological conditions and slow infiltration rates Žhydrological soil group C.., abandoned fields: 63, Žequivalent to sagebrush with grass understory and a ground cover of 30–70%. and Stipa zones:

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Fig. 8. Study area with the reach of Rambla Salada, the bank gullies and the present Ž1997. land use.

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Table 4 Ža. Cross-tabulation of land use and gully head activity classes. x 2 s Pearson’s Chi-square statistic; df s degrees of freedom. Žb. Gully head activity expressed as a percentage of each land use type. Žc. Cross-tabulation of land use and gully head activity classes adjusted for control measures, i.e., ‘moderately’ and ‘non-active’ counts with control measures are included in ‘active’ class. This improves the significance level Land use

Very active

Active

(a) Absolute counts Stipa 21 116 Wheat 8 32 Almonds 7 10 Abandoned 3 8 Vineyard 0 0 Total 39 166 x 2 s18.7; df s12; significant at 0.1 level (b) Percentage of total Stipa 6.6 Wheat 10.5 Almonds 24.1 Abandoned 9.1 Vineyard 0

36.4 42.1 34.5 24.2 0

(c) Absolute counts adjusted for control measures Stipa 21 116 Wheat 8 35 Almonds 7 15 Abandoned 3 8 Vineyard 0 0 x 2 s 25.6; df s12; significant at 0.02 level (d) Percentage of total Stipa 6.6 Wheat 10.5 Almonds 24.1 Abandoned 9.1 Vineyard 0

36.4 46.1 51.7 24.2 0.0

Moderately active 109 23 5 12 1 150

Not active

Total

73 13 7 10 0 103

319 76 29 33 1 458

34.1 30.3 17.3 36.4 100

22.9 17.1 24.1 30.3 0

100% 100% 100% 100% 100%

109 20 4 12 1

73 13 3 10 0

319 76 29 33 1

34.2 26.3 13.8 36.4 100

22.9 17.1 10.3 30.3 0.0

100% 100% 100% 100% 100%

63–80, Žequivalent to sagebrush with ground cover of 20–50%.. The deduced curve numbers confirm the proposed hierarchy in runoff, since higher numbers indicate higher runoff potential. Ground surfaces in almond groves are kept bare all year round, while wheat covers the soil part of the year. Therefore, it is expected that more runoff is generated from almond groves over a whole year period. Abandoned fields are partly covered with vegetation all year round which reduce the runoff production. No runoff measurements for this area are available but these runoff patterns agree well with observations from a Mediterranean environment in north-eastern Morocco by Laouina Žpersonal communication.. He reported runoff generation from cultivated land in 100% of all rainfall events, after 10 mm of rainfall while 20 mm of rainfall were needed on abandoned fields with a 30% vegetation cover. Only 7 mm rainfall was sufficient to produce runoff on cultivated fields in 50% of the events. The greatest uncertainty is

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associated with the Stipa zones. These are reported as high runoff production areas in rangeland ecosystems ŽNicolau et al., 1996.. However, high runoff production in Stipa zones is particularly evident in areas with bedrock close to the surface, while the slopes along the Rambla Salada are formed in unconsolidated sediments. Moreover, it is likely that runoff is discontinuous in Stipa zones. Bergkamp et al. Ž1996. observed runoff to infiltrate at the vegetated rim of terracettes during rainfall simulation experiments in matorral in Southeast Spain. Terracettes were also present in the stipa zones near the Rambla Salada, therefore, Stipa zones generate probably less runoff compared to cultivated fields. The erosion of a gully head in a certain lithology depends strongly on the volumes of runoff that are generated in its contributing catchment area. Because runoff generation is also affected by land use, certain combinations of lithology and land use may show predominantly active or not active gully heads. Fig. 9 shows that the combination of marls and wheat fields have the highest percentage of very active gully heads Ž38.9%., followed by the combination of Quaternary infill and almonds Ž26.9%.. It should be noted that the combination of marls and almonds, each of which scored highest for very active gully heads, did not occur in the research area. The combinations with conglomerates and gravel show less very active gully heads. However, because the number of gully heads is low in such areas, the distributions have to be regarded with caution.

Fig. 9. Gully head activity for all occurring combinations of lithology and land use. The combination of marls and wheat fields include the highest percentage very active gully heads Ž39%., followed by the combination of Quaternary infill and almonds Ž27%.. The areas with conglomerates Žcongl.. and gravel have less very active gully heads. The numbers between brackets indicate absolute numbers of surveyed gully heads.

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4.4. Orientation and channel bends Other factors that seem to affect the spatial distribution of gully head activity are orientation and the position along the channel. Slope aspect can create contrasts between dry south-facing slopes with thin poorly vegetated soils and more humid north-facing slopes with thicker, well vegetated soils. In our study site, 88 gully heads were clearly south-facing and 86 north-facing. The Chi-square test did not reveal a dependency between gully head activity and slope aspects Ž p s 0.21.. However, a strong dependency was found for gully head activity and channel bends ŽRambla Salada or tributary gullies.. The 63 gully heads that clearly connected to outer bends of channels showed an overall greater activity than the 20 gully heads that clearly connected to inner bends ŽChi-square: p s 0.0015.. The rest of the gully heads Ž375. were located in more or less straight sections. One reason is probably related to the larger contributing catchment areas in outer bends. Furthermore, no alluvial fans easily develop on channel junctions in outer bends because the sediments are carried away with the mainstream. Therefore, less aggradation occurs in gully channels connected to outer bends than those joining inner bends. This implies that back-filling of channels and eventually, infilling of gullies or slowing down of gully head erosion is less likely to occur in gullies connected to outer bends than in those connected to inner bends. 5. Rates of gully head erosion As mentioned above, the mapping, and classification of gully-activity showed that in the selected area, one active or very active gully head occurs per 17 m of Rambla length. The average sediment production per active gully over the 2-year period amounted to 4.0 m3 of sediment per year ŽVandekerckhove et al., in preparation.. There was a large difference in mean sediment production between the 2 years, i.e., 7.3 m3 Ž1997. and 0.9 m3 Ž1998.. Rainfall amount is one factor that could explain the difference partially since the first year was wetter than the second Ž409 mm in 1997 and 304 mm in 1998.. However, Vandekerckhove et al. Žin preparation. argue that the large contribution of a few gully heads due to failure of tension cracks and the unequal distribution of rainfall in the study area are the main reasons for the different eroded volumes for the 2 years. Because this study is designed as a long-term project, the average of 4.0 m3 is used here as a tentative figure. When more data become available, in time, the figure will be updated. If this mean eroded volume is applied to the active and very active gullies of this study area, an annual volume of 4.0 m3 of sediments enters the Rambla every 17 m solely due to gully head activity. The entire Rambla reach of 3500 m includes 205 active and very active gullies, which would be equivalent to an annual sediment production of 820 m3. Although eroding gully heads are point sediment sources in the landscape, they can also be viewed as diffuse sediment source when expressed in tons per hectare, which allows comparison with other processes of erosion. The extent of the area affected by bank gullies essentially includes the sedimentary fill of the basin, which is characterised by fairly low slopes, i.e. - 10%, cultivated or abandoned lands with some land occupied by matorral or alfa grass. The upslope boundary of the sedimentary fill coincides

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approximately with a sharp increase in slope and with a change in land cover from cultivated to matorral or forest. This area was determined by using digitised 1:25,000 topographical maps and 1:20,000 aerial photographs. For the study area, a surface area of 866 ha was calculated, which could be affected by bank gullying. Thus, the Žspecific. erosion rate of gully heads in this area amounts to 820r866s 0.95 m3 hay1 yy1 . To convert the volume to a mass, a mean dry bulk density of 1.3 ton my3 is used ŽRebeiro-Hargrave, personal communication., which results in 0.95 = 1.3 s 1.2 tons hay1 yy1 . 6. Extrapolation of results to the catchment of the Puentes Reservoir The selected reach of the Rambla Salada drains into the Puentes Reservoir ŽFig. 1.. which was built in 1884 with a capacity of 13 hm3. Sanz Montero et al. Ž1996 and personal communication. estimated the total catchment area of the reservoir at 1042 km2 and the mean long-term annual sediment yield at 2.02 tons hay1 yy1 . The analyses above show that the erosion rate of the gully heads is of the same order of magnitude Ž1.2 tons hay1 yy1 .. This indicates that gully heads are important sediment sources but in order to evaluate their potential contribution to the infilling of the reservoir, a catchment wide estimate of sediment volumes has to be made. In addition, the sediment delivery ratio ŽSDR. for the catchment has to be known. Recently, Trimble Ž1999. showed that sediment yields at the outlet of a basin can behave quite independently from the erosion rates within the basin for long periods of time. We have no detailed information about the sources and sinks in the catchment. The SDR for sediments released by gully head erosion is probably high because these sediments enter the channel system directly and no floodplains are present along the channels. No indications of channel aggradation were observed but it is unlikely that the ephemeral channels are in a steady state. Therefore, erosion from sources, i.e., gully heads, cannot be directly linked to sedimentation in the reservoir but provides an indication of the relative importance of the erosion process. The selected reach of the Rambla Salada is representative for a larger section of the same Rambla and also for other Rambla sections in the catchment. These other sections all occur in similar type of settings, i.e., low-angled sedimentary basins that are largely cultivated or abandoned. These Rambla sections and their surrounding sedimentary basins could thus be identified by using aerial photographs. Digitising this information on topographical maps allowed estimation of their total length and surface areas for the whole catchment. Fig. 10 shows the sedimentary basins and the drainage sections where bank gullying is very active. Three areas are selected which are comparable to the training area in terms of lithology, slope, and land use, i.e., sedimentary basins I, II, and III. The total length of the Ramblas that traverse these areas is 41,165 m and the total area 12,760 ha ŽTable 5.. Using the assumption of one Žvery. active gully head per 17 m of Rambla, this would imply that 2421 Žvery. active gully heads produce 9686 m3 yy1 or 0.99 ton hay1 yy1. These values are a little lower than that for the training area but still rather similar and the same conclusions can be drawn for the bank gully erosion rates at the catchment scale, i.e., bank gully extension as a single process provides sediment to the reservoir at

D.J. Oostwoud Wijdenes et al.r Catena 39 (2000) 147–167

Fig. 10. Drainage basin of the Puentes Reservoir. The sedimentary basins I, II, and II are indicated.

163

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Table 5 Summary of sediment production by bank gully heads in the sedimentary basins I, II, and III. For location of sedimentary basins, see Fig. 10 Sedimentary basins I

II

III

IqIIqIII

12,073 25,442

316 7288

371 8435

12,760 41,165

One actiÕe gully head per 17 m Rambla Number of Žvery. active gully heads Sediment production Žm3 yy1 . a

1497 5987

429 1716

496 1983

2421 9686

Bulk density:1.3 ton m y 3 Sediment production Žton yy1 .

7783

2231

2578

12,592

3.7

1.1

1.2

6.0

0.64 12

7.06 0.3

6.95 0.4

0.99 12

Surface area Žha. Length of Ramblas Žm.

Area draining to Puentes ReserÕoir: 104200 hab Sedimentation rate in Puentes ReserÕoir: 210484 ton y y 1b EquiÕalent to soil loss in catchment: 2.02 ton ha y 1 y y 1b Contribution from bank-gully head erosion to reservoir sedimentation Ž%. Sediment production Žton hay1 yy1 . Ždiffusive. Proportion of Total catchment area Ž%.

a Assuming sediment production of 4.0 m3 yy1 per Žvery. active gully head Žbased on monitoring of 46 active gully heads over a 2-year period, Vandekerckhove et al., in preparation.. b Data from Sanz Montero et al. Ž1996..

a rate which is of the same order of magnitude as the rate at which the reservoir is silting up. Thus, bank gullying is a major source of sediment from the intermountain sedimentary basins. The total proportion of gully head erosion in the sedimentary basins I, II, and III is 6% of the annual sediment yield of the reservoir ŽTable 5.. Other sources of sediment have not been quantified but probably include channel side walls, channel beds, and hillslopes within and outside the three sedimentary basins. Poesen et al. Ž1996. and Vandekerckhove et al. Ž1998. report that ephemeral gullies are major sediment sources on hillslopes in Southeast Spain. However, ephemeral gullies occur only in a relatively small area of micaschist in the southern part of the catchment of the Puentes Reservoir. The selected sedimentary basins I q II q III where the bank gully heads occur occupy 12% of the total catchment area ŽTable 5.. On the basis of their aerial extent, the sediment contribution of the gully heads seems large if we assume that channel sidewalls, channel beds, and hillslopes also contribute sediment to the channel. It follows that the sedimentary basins are major sources of sediment that fill up the reservoirs. Moreover, because the upstream Valdeinfierno reservoir ŽFig. 10. captures most of the sediment of its subcatchment, the relative sediment contribution of the sedimentary basin I, II, and III to the Puentes Reservoir will even be greater.

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The rest of the catchment includes mountainous terrain which is well vegetated and smaller areas with sedimentary fills. Bank gullies occur at some Ramblas particularly in the area north-west of the Valdeinfierno reservoir. The steeper hillslopes provide probably less sediment because they are better protected by vegetation. 7. Discussion and conclusions The upslope expansion of gullies from ephemeral channel banks is typical for many semi-arid areas. Our study of these systems along the Rambla Salada revealed a spatial relationship between gully head activity and lithology and land use. Marls and almond groves locate relatively the most active gully heads, while conglomerates and abandoned land have less. This implies that almond groves Žwhich showed a large aerial expansion in the last decades. are very important runoff generating areas Žre.activating bank gully heads, and that these gully heads are potentially important sediment sources in cultivated fields. A proposed runoff hierarchy was confirmed by the runoff curve numbers. Spatial relationships between gully head erosion and environmental factors can be used to develop gully erosion risk maps as a practical tool for sustainable land-use planning. The research also showed that simple earth dams can effectively slow down gully head erosion. Care must be taken that the runoff accumulating behind the walls is not diverted to the gully sides, and in this way, creating a new gully head. If the water can infiltrate behind the walls, a densely vegetated buffer zone can develop which will help to stabilise the gully head. As a disadvantage of water harvesting behind a gully head it might be argued that the nearby incision of the gully creates a steep hydraulic gradient and may lead to piping ŽCrouch, 1976.. However, this thread is reduced by the water uptake by plant roots. Moreover, gully head erosion by collapsing pipes occurs in most materials at lower rates than erosion by surface runoff. Extrapolation of the rates of gully head erosion to the whole catchment, by using the density of gullies and an average annual retreat rate, showed that bank gully head erosion produces sediment at a rate which is of the same order of magnitude as the long-term estimated sedimentation rate in the Puentes Reservoir. Within 12% of the catchment area, 6% of the total annual sediment yield is produced by bank gully head retreat only. Although the figures are tentative, they indicate that bank gully erosion is a major source of sediment that is potentially filling up the reservoirs. Soil conservation measures aimed at controlling bank gully heads may reduce the sediment yield significantly. In addition, if the gully control measures also reduce channel flow Žfor example, by increasing infiltration., gully walls will also stabilise and the sediment output will be further reduced. Because only the active and the very active gully heads were included in the calculations and not the moderately active ones, the estimates can be regarded as conservative. Moreover, three major sedimentary basins were selected within the catchment. Locally, gully head erosion may also occur in small sedimentary deposits elsewhere in the catchment, which is not accounted for in the estimates. Finally, research on gully head activity should be expanded to new areas to test the spatial relationships on other lithologies and land covers in the Mediterranean. In addition, more research is needed into the runoff-production potential of the various

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land-use types in the Mediterranean. Enlargement of fields and the introduction of different vegetation or crop cover types have changed the runoff potential dramatically over the last decades. These changes are closely linked to gully head activity. It will also be useful to include more topographical attributes in the analysis such as slope, catchment size, and the position of the bank-gully head with respect to the main channel.

Acknowledgements The research for this study was carried out as part of the MEDALUS III ŽMediterranean Desertification and Land Use Project. collaborative research project. MEDALUS III was funded by the European Commission Environment and Climate Research Programme Žcontract: ENV4-CT95-0118, Climatology and Natural Hazards. and this support is gratefully acknowledged. Laurent Beuselinck and Patriek Bleys are thanked for their assistance with fieldwork. Andrew Rebeiro-Hargrave ŽKings College London. kindly provided the data on the bulk density.

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